With respect to an accelerator coil incorporated in an electromagnet of an accelerator, in order to accelerate charged particles up to an intended kinetic energy, it is required that an axis and an up-down center plane of the accelerator coil coincide highly accurately with a reference axis and a reference plane to be described later, respectively. For example, in an accelerator such as a synchrocyclotron or the like, a split solenoid coil in which two coils are paired is incorporated. For the split solenoid coil, it is required to reduce, as much as possible, positional errors of three types according to: an inclination of the coil axis from the reference axis; a horizontal displacement of the coil axis from the reference axis; and a displacement of an up-down center position between the two coils from the reference plane. In many cases, the reference axis and the reference plane are set on a highly accurately-fabricated iron yoke of a cryostat or electromagnet, to be described later.
In Patent Document 1, there is described a magnetic field measuring apparatus and method for causing a structure center of a solenoid electromagnet used for convergence, etc. of charged particles in an accelerator, to be matched to the magnetic center line of the solenoid electromagnet. According to the magnetic field measuring apparatus of Patent Document 1, as shown, for example, in FIG. 1 of Patent Document 1, vertically and horizontally-directed two magnetic-field sensors (magnetic field measuring elements) are placed on a rotatable disk, so that an inclination of a coil axis of a solenoid coil, etc. and a horizontal displacement of the coil axis, relative to a reference axis, are corrected based on a magnetic field characteristic measured by the magnetic field sensors while the disk is being rotated.
Meanwhile, in Patent Document 2, there is described a magnetic field measuring method of measuring a three-dimensional magnetic field distribution of a magnetic instrument such as a deflection yoke apparatus, a transformer or the like. According to a magnetic field measuring apparatus shown in FIG. 12 of Patent Document 2, a magnetic field sensor (magnetic field measuring element) that comprises a Hall element and is placed at an angle of π/4 or 3π/4 on an axis (sensor rotation axis) parallel to the coil axis, is rotated around that sensor rotation axis, to thereby measure three magnetic field components Bx, By, Bz.